The present invention is directed to a computer system for modeling the parameters of a lithographic process and flexibly providing graphic displays useful in optimizing the lithographic process. In particular, the present invention is directed to a system that determines the optimum range of exposure and focus settings for reliably manufacturing integrated circuits within desired feature width tolerances.
Lithography is the process used for patterning semiconductor wafers in the manufacture of integrated circuits. Lithographic performance is determined by the measurement and control of two key parameters: critical dimension (CD)--the size of critical features on the pattern, typically line width--and pattern registration (PR)--the relative placement of pattern features with respect to one another. As feature sizes shrink and integrated circuit complexity increase there is a growing need for improved methods to analyze the CD and PR data in order to optimize and control the manufacturing process.
Variations that may occur in the lithographic process may be divided into spatial (different patterns printed at different locations at approximately the same time) and temporal (the same pattern at the same location printed at different times distributions. The appropriate time scale is determined by the frequency of variation in the process. In the extremes, a perfectly stable process is characterized entirely by the spatial distribution, whereas a wildly unstable process is characterized by the temporal distribution. More typical of existing lithographic capability is that both distributions must be considered. For a reasonably stable process, a batch of wafers patterned during a single shift exhibits mostly spatial variation, whereas temporal variations occur over several shifts.
The spatial and temporal variations have controllable components that may be traced to adjustments in lithographical parameters and uncontrollable components. It is an object of the present invention to provide an analysis of variations so as to improve the manufacturing process by permitting optimization of controllable parameters on the manufacturing equipment. By optimizing the manufacturing parameters such a the choice of exposure time, film thickness, bake temperature, development rate, etc., present equipment can operate closer to the target operating point and reduce spatial and temporal variation. Since new improved equipment is not required, parameter design is the least expensive approach to improving the manufacturing process.
Systems are available which can receive raw data from metrology and lithographic tools. However, methods have not been available for using the data for determining the optimum range of exposure and focus settings for producing integrated circuits within desired feature width limits. It is an object of the present invention to provide a system that can flexibly display graphs of the relationship between focus, exposure and feature width to improve the usefulness of the data. It is a further object of the present invention to provide a model of the critical dimension data thereby reducing that data to equations which can be quickly and efficiently analyzed and interpreted by a computer. It is a still further object of the present invention to provide rapid analysis of the critical dimension data so as to permit real time control of the lithographic process.